This invention relates to solid state imaging circuitry and systems and to methods for operating the circuitry and system.
To better understand the invention and a problem resolved by the invention reference is made to FIG. 1 which depicts a large two dimensional (matrix) array of image sensing elements (pixels) arranged in rows and columns. Assume, for purpose of illustration, that the array is comprised of 100 rows and 100 columns with one picture element (pixel) located at the intersection of each row and column. Assume further that bursts of light pulses, representing the information to be sensed, are to be detected (sensed) by the matrix array. For purpose of illustration assume that there are bursts of light pulses with the repetition rate of the light pulses within a burst of pulses being 33,000 pulses per second (i.e., the time interval, or period, between light pulses within a burst of pulses is 30 microseconds, corresponding to a frequency of approximately 33 KHz). To be able to sense the occurrence of each individual light pulse sensed by any one of the 10,000 pixels comprising the 100xc3x97100 pixel matrix array, a conventional (prior art) matrix array has to be operated at a rate of 330 MHz. (i.e., 33 KHz times the 10,000 pixels in the array).
Consequently, the matrix array has to be operated at a high frame rate (33 KHz) and the information has to be sensed and processed at a very high video bandwidth. Operating the array at such high frame rate and bandwidth is problematic. This problem is further aggravated when it is desirable and/or necessary to use a larger matrix array (e.g., a 256 by 256 array of photo elements). Using known arrays of photo elements and known techniques for operating these arrays require very high operating clock rates and very high speed of data transmission which is difficult, if not impossible, to effectuate.
The problems associated with known prior art arrays of image sensing elements and the operation of these arrays are overcome in systems embodying the invention.
The problems associated with the known prior art arrays of image sensing elements and their operation are overcome in systems embodying the invention by using a novel array of xe2x80x9csmart pixelsxe2x80x9d embodying the invention.
The pixels used to practice the invention are denoted as xe2x80x9csmartxe2x80x9d in that each pixel of an image sensing array embodying the invention includes the ability to detect (sense) the receipt of optical pulses during a sampling interval and to generate and store within each pixel an indication of the number of optical pulses detected or sensed by the pixel during the sampling interval (frame time).
In one embodiment, each pixel has associated with it a photodetector to sense an incident light pulse and an analog voltage counter to count and store the number of light pulses detected by each pixel, during a sampling interval. The analog voltage counter generates a voltage whose amplitude corresponds to the number of pulses detected by the photodetector during the sampling interval. That is, each pixel is designed to count the number of light pulses detected by the pixel during a xe2x80x9csampling intervalxe2x80x9d and to store a voltage corresponding to the count. Following each sampling interval, the data sensed and stored by each pixel is read out and the analog voltage counter is reset to an initial condition. Since each pixel includes means for storing a voltage corresponding to the number of light pulses sensed by the pixel, the pixels can be read out at a moderate frame rate.
In another embodiment of the invention, the number of light pulses detected during a sampling interval are stored in a digital counter.
Image sensors embodying the invention are comprised of an array of smart pixels with each smart pixel including a photodetector, for detecting optical pulses, and circuitry for processing the electrical signal generated by the photodetector. The signal processing circuitry is designed and tailored to respond to optical pulses in order to count only those pulses having a predetermined characteristic and means for counting and storing the detected light pulses.
Image sensors embodying the invention may also include circuitry for operating the array such that each pixel of the array detects incoming light pulses for a sampling interval with the information acquired by each pixel during a sampling interval being subsequently read-out during a read interval.
The xe2x80x9csmart pixelxe2x80x9d concept greatly reduces the video bandwidth and raw data processing rate required to detect and locate low intensity light pulses (lasers) in a wide field-of-view.
In one embodiment, each pixel is provided with a photodetector, a transimpedance amplifier and a high-pass circuit to detect and count short optical pulses superimposed on a bright background. Each smart pixel includes counting means for counting multiple optical pulses to provide a means for discriminating bursts of optical pulses from cosmic rays and other single event noise sources.
The pixel array may be read out by conventional x-y addressing of each pixel. In one embodiment using an analog voltage counter, the readout process consists of reading an analog voltage level that is related to the number of pulses detected since the last time the pixel was read out and reset. When the output signal voltage has been read, the voltage level is reset to a reference value representing zero detected pulses.